Concentric coil sheath heating unit



Jan. l0, 1950 L. M. DALY coNcEN'rRIc con. sHEA'm HEATING UNIT Filed June9, 1948 Inventor Loretta M. Daly,

by m 10m He* Attorney.

UNITED STATE Patented. l0, i950 2,494,333 CONCENTRIC COIL SHEATH HEATINGUNIT Loretta M. Daly, Pittsfield, Mass., assignor to General ElectricCompany, a corporation of New York Application June 9, 1948, Serial No.31,978

9 Claims. (Cl. 201-.67)

My invention relates to electric heating units, more particularly toelectric heating units of the sheath wire type, and has for its objectthe provision of an improved heater of this character.

More specically, my invention relates to electric heating units of thesheath wire type in which concentric coils of equal or unequalresistance enclosed by the metallic outer sheath, are embedded in apowdered heat refractory insulating material which is compacted to ahard dense mass so that it may readily conduct heat from the coils tothe sheath.

Heretofore in order to secure a high temperature output from a givenlength of heatentwo single coil metallic sheath units were placed sideby side within a third sheath of oval cross-section and the resultingunit was subjected to a swaging process to compress the unit to acircular cross-section. The coils were then connected in series. In theswaging process, the two inner sheaths were distorted within the newconilguration, becoming two lobes filling the now circular outer sheathand in contact with each other along a diametric plane of the outersheath.

Using this method, even though the two inner units were packed andswaged before lbeing placed within the outer sheath, the swaging of theouter sheath often resulted in decreased electrical clearances Withinthe inner units, so that they would break down under high potentialtest. Such faulty units could not be repaired and their scrapping causedhigh manufacturing losses.

My invention is less costly since the two inner sheaths used in theformer method are eliminated. Units constructed by the former methodwere less eiiicient in operation since after swaging the twoinnersheaths did not completely fill the circular area within the outersheath. Instead, it was found that definite voids or air spaces resuitedbetween adjacent inner surfaces. These voids caused a heat transfer lossfrom the single coil units tothe sheath, increasing the coil temperaturerequired to produce a given temperature of outer sheath. Also, myinvention results in increased insulative clearance since for equalcross-section, the volume of the inner metallic sheaths of the formerdesign has been replaced by insulation material.

In my invention a plurality of coils are mounted concentric with anouter sheath and suspended between stepped concentric insulatedterminals using ceramic spacers between two adjacent coils as well asbetween the outer coil and the sheath. By vibrating powdered insulationinto the end capped outer sheath, the insulation is caused to fillcompletely the voids in and between the coils and between the coils andthe sheath. The ends of the sheath are capped with the terminal membersfastened securely to the caps and the whole unit -is subjected to adiameter reducing process such as swaging. This reducing process servesto compact the insulation about the coils even more densely to effect amaximum heat transferability as well as maximum electrical insulation.

During the reduction process, the whole unit becomes elongated as wellas reduced in outside diameter, and the encompassed coils are lengthenedproportionately without changing the relative position of the coils andthe sheath. Since the reduction process in my invention is a uniformlyundistorted reduction in diameter, the possibility in the former methodof diminution of insulative or electrical clearance within the innersheaths due to distortion during the diameter reduction process, doesnot exit. The heat loss in the former method caused by voids within theouter sheath due to the use of inner sheaths is eliminated since in myinvention, there are no inner sheaths. The same or greater value ofelectricai resistance, therefore, may be included in a unit of equallength and diameter without these limitations and disadvantages of theformer method.

My invention is adapted for use as a cpmpact unit for the generalpurposes of sheath type heaters, or as an immersion or insertion typeheater. In the general purpose heater, by using different resistancesand providing a means for externally changing the connection between thecoils, a selective heat feature is included in my invention allowing achoice of several overall heating values in one unit. Either immersiontype or insert typeheaters are obtained by eliminating the connection atone end of the unit and, internally connecting the coils in series. Y

My invention will best be understood by reference to the accompanyingdrawing, in which Fig.

2 is a sectional view of a terminal filled with insulation and ready tobe swaged. Fig. 1 is a sectional view of the complete heating-unit; Fig.3 is a view of a terminal for a Iheater in'which two coils are seriesconnected; Fig. 4 is an alternate construction of an immersion or inserttype heater.

Referring to Fig. 2, one embodiment of my invention in which asub-assembly of a terminal l member comprising two concentric terminalsor conductors is shown, a metallic terminal rod I is inserted within ametallic tubular terminal 2 po- SitiQiled vertically with the innerterminal pro- 3 jecting at one end. The cup washers I and 4 position thetwo terminals concentrically. The voids are illled with powderedinsulation material l through apertures provided in the upper cup washer3 for this purpose. During the filling process the sub-assembly iscontinually vibrated to insure compact nlling of the voids between theterminals. The terminal members are cut back at both ends in steppedrelation to provide connection surfaces. A second sub-assembly is formedby securing the coils 1 and 8 respectively between equal diametersurfaces of the two terminal members l. For securing such members,

welding is preferred to crimping to avoid danger of distortion oftubular terminals. Ceramic spacers (not shown) are mounted at intervalsbetween the two coils to insure maintenance of their substantiallyconcentric relative position during the subsequent insulation illlingoperation. The second sub-assembly of terminals and coils is mountedwithin the vertically disposed outer sheath 9 of the unit and heldconcentric with the sheath by means of end washers and cross pins (notshown), and ceramic spacers (not shown) suitably disposed between theouter coil and the sheath. Both sets of ceramic spacers are used to holdcoils and sheath concentric while assembling in the outer sheath andduring the following insulation filling operation. The spacers may breakup and mingle with the insulation material during the subsequentdiameter reduction proc ess. The unit is then vibratingly lled with powdered insulation through apertures provided in the upper washer providedfor the purpose and the whole unit is reduced in diameter to compact theinsulation to a hard dense mass so the insulation will conduct heatreadily from the resistance elements to the outer sheath. The outersheath is cut away at each end to the desired length. The end washersused in the filling and reduction operation, are replaced by insulatingwashers In and terminal Wires Il are clamped tightly to the contactsurfaces of the terminals by means of collars I2 with set screws I3 totighten against the terminal sheath. Insulating washers i4 are slid overthe ends of the terminal rods and suitable terminals i5 are attached tothe rods.

When more than two concentric coils are desired, the voids of theconcentric terminals are vibrationally filled simultaneously in asimilar manner to that described above. The terminals are cut back ateach end in stepped relation and the surfaces of corresponding diameterare connected respectively by the concentric resistance coils.

My invention allows several values of resistance to be obtained from thetwo coil units merely by changing the external connections. Either coilmay be used individually or the unit may be used with the coils inseries connection or in parallel connection. If two different values ofcoil resistance are used, the number of combinations is increased. In athree or more concentric coil unit, many values of unit resistance areavailable by the use of unequal individual coil resistances and shuntand series interconnections.

Fig. 3 is a view of a, one piece terminal for use where two coils are tobe connected in series only.

Fig. 4 is a view of a two-coil immersion or insertion type heater withone piece terminal I6 for series connection betweenV coils. Cap I`1completely covers the immersion end of the unit and is tightly attachedto the outer sheath'around a recessed portion of the outer surface oi'the sheath to waterproof the unit.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. An electric heating unit comprising a metallic sheath, a plurality ofhelical resistance elements of different diameters in said sheathextending lengthwise thereof in substantially concentric relation witheach other, a compact mass of powdered heat conductive'electricalinsulation material in said sheath embedding said resistance elements, aterminal member assembly extending into at least one end of said sheathand having its inner end embedded in said insulation material, saidterminal member assembly comprising an outer tubular terminal and aninner terminal rod, electrical insulating material in said tubularterminal' securing said terminal rod in substantially concentricrelation therewith, and electrical connections respectively between theinner ends of said terminals and said' resistance elements.

2. An electric heating unit comprising a metallic sheath, a plurality ofsubstantially concentric helical resistance elements, a powdered heatconductive electrical insulation material, said resistance elementsbeing embedded in the said insulation material within the sheath, saidinsulation material being compacted to a hard dense mass to insure thatsaid mass conducts heat readily from the said resistance elements to thesaid sheath, and a terminal member in each end of the said sheath, eachof said terminal members comprising electrically insulated substantiallyconcentric terminals in each end of said sheath connected respectivelyto said concentric resistance elements, said terminalmembers protrudingfrom each end of said sheath, and said terminals being cut back fromtheir ends in stepped relation to form terminal surfaces for theconnection of the said unit in an electric circuit.

3. An electric heating unit comprising a metallic outer sheath, aplurality of substantially concentric helical resistance elements andpowdered heat conductive electrical insulation material,

said resistance elements being embedded in said insulation material,said insulation material being compacted to a hard dense mass to insurethat said mass conducts heat readily from the said resistance elementsto the sheath, and terminal members each comprising electricallyinsulated substantially concentric terminals secured respectively to theends of said resistance elements, said terminal members projectingbeyond said outer sheath to form terminals for the connection of thesaid unit in an electric circuit.

4. An electric heating unit comprising a metallic outer sheath, aplurality of substantially concentric helical resistance elements andpowdered heat refractory insulation material, said resistance elementsbeing embedded in the said insulation material, said insulation materialbeing compacted to a hard dense mass within said sheath to insure thatsaid mass conducts heat readily from the said resistance elements to thesaid sheath, and terminal members each comprising electrically insulatedsubstantially concentric termlnals, said terminals being cut back yfromeach end in stepped relation to form connection contact surfaces, saidterminals at their inner ends being secured respectively to ends of thesaid concentric resistance elements, said terminal members projectingbeyond said outer 'sheath to form terminals for the connection oi.' 75the said unit in an electric circuit.

5. An electric heating unit comprising va metallic outer sheath, twoconcentric helical resistance elements, powdered heat refractoryelectrical insulation material, said resistance elements being embeddedin the said insulation material within the said sheath and saidinsulation material being compacted to a hard dense mass to insure thatsaid mass conducts heat readily from the said resistance elements to thesheath, said unit having terminal members projecting beyond either endof the said outer sheath, one of said terminal members comprising asub-assembly of electrically insulated concentric terminals, saidconcentric terminals being out back in stepped relation at their innerends and respectively secured to the ends of the said concentricresistance elements, the outer ends of the said terminals` being cutback in stepped relation to form terminal contact surfaces fortheattachment of lead wires, the other of said terminal members beingcylindrical and having its inner end secured to the one of saidresistance elements of larger diameter, said other terminal having acylindrical projection of smaller diameter to which is secured the otherof said resistance elements, said other terminal member providing at itsouter end a single terminal'for the attachment of the unit in anelectric circuit.

6. An electric immersion heating unit comprising a metallic sheath, aplurality of concentric helical resistance elements, powdered heatrefractory insulation material, said resistance elements being embeddedin said insulation material within the said sheath, said resistancematerial being compacted to a hard dense mass to insure that said massreadily conducts heat from the said resistance elements to the sheath, aterminal member in one end of said sheath comprising a sub-assembly ofelectrically insulated concentric terminals, said terminals being outback from each end in stepped relation to form connection contactsurfaces, and the said connection surfaces at the outer end of the saidv terminal member forming terminals for the connection of the unit in anelectric circuit, the said contact surfaces at the inner end of the saidterminal member being respectively secured to the ends of the saidconcentric resistance elements the opposite end of the said resistanceelement of larger diameter being secured to a cylindrical terminal, saidcylindrical terminal having a concentric cylindrical projection ofsmaller diameter to which is secured the inner of the said concentricresistance elements, said unit being provided with a cap completelyenclosing the end of the unit and secured tightly to a recessed endportion of the outside surface of the said sheath to waterproof theunit.

7. A method of forming a concentric coil electric heating unitcomprising a metallic outer sheath, a sub-assembly of concentric helicalresistance elements and terminal members and compacted powdered heatrefractory insulation, said resistance elements being embedded in thesaid powdered insulation material within the said sheath, said terminalmembers protruding from each end of the said sheath to provide terminalsfor connection of the said heating unit in an electric circuit, saidmethod comprising forming a secondsub-assembly of each of said terminalmembers by vibratingly lling the voids between concentric terminalswiththe said powdered refractory insulation material to insure that therelative positions of the said terminals be maintained during subsequentoperations, cutting back tion to provide connection contact surfacesforming the said sub-assembly by suspending the said.-

concentric coils between the said terminal members and securing the endsof the coils respectively to the said concentric connection surfaces atthe inner ends of the said terminal members, mounting the saidsub-assembly concentrically within a metallic outer sheath with theterminal members projecting substantially equally at both ends of thesaid outer sheath, vibratingly filling the voids within the said outersheath with said powdered refractory insulation material and, by meansof a diameter reducing process applied to the said outer sheath,compressing the insulation to a hard compact mass so that the said massmay readily conduct heat from the said resistance elements to the saidsheath.

8. A method of forming a concentric coil electric heating unitcomprising a sub-assembly of two concentric helical resistance elementsattached to terminal members, compacted powdered heat refractoryinsulation, and a metallic outer sheath, said resistance elements beingembedded in said compacted powdered insulation within the said outersheath, said terminal members protruding from each end of the saidsheath to provide terminals for connection of the said heating unit inan electric circuit, said method comprising forming a second subassemblyof one of the said terminal members by vibratingly filling the voids inand between concentric terminals with a powdered refractory insulationmaterial to insure that the relative positions of said terminals bemaintained during subsequent operations, cutting back both ends of thesaid terminals in stepped relation to form connection contact surfacesat one end for connection of the said unit in an electric circuit and atthe other end for attachment to the concentric helical resistanceelements, the other of said terminal members comprising a cylindricalbar with a concentric cylindrical projection, the diameters of said barand said projection being equal to the diameters of the terminals of thesaid second sub-assembly, forming the said sub-assembly by suspendingthe said helical concentric resistance elements respectively between thesaid equal diameter surfaces of theA two said terminal members andsecuring the ends of the saidhelical elements to the said surfaces,mounting the whole concentrically within a metallic outer sheath withthe terminal members projecting substantially equally at both ends ofthe said outer sheath, vibratingly filling the voids within the saidouter sheath with the said powdered refractory insulation material andby means of a diameter reducing process applied to the said outersheath, compressing the insulation to a hard compact mass so that thesaid mass may readily conduct heat from the resistance elements to thesheath.

9. A method of forming a concentric coil immersion or insert typeelectric heating unit comprising a sub-assembly of concentric helicalresistance elements and terminal members, powdered insulation material,an outer metallic sheath, and a waterproof end cap, said resistanceelements being embedded in the said insulation material within the saidsheath, said material being compacted to a hard dense mass so that saidmass may readily conduct heat from the said elements to the said sheath,one of the said terminal members protruding from the said outer sheathto provide terminals for connection of the anima od comprising formingthe said sub-assembly by vibratingly filling the voids of the said oneterminal member with the said powdered refractory insulation material toinsure that the relative positions of the said terminals be maintainedduring subsequent operations, cutting back the said terminals from eachend in stepped relation to form yconcentric contact surfaces, the otherof the said terminals comprises a cylindrical rod with a. concentriccylindrical projection at one end the diameters of the surfaces of saidrod and said projection being substantially equal to the diameters ofthe terminals of the said one terminal member, suspending the saidhelical elements respectively between the said equal surfaces andsecuring the ends of the said helical elements to the said surfaces,mounting the whole concentrically with the said outer sheath with theterminal members projecting at both ends of the said outer sheath,vibratingly filling the voids within the said outer sheath with saidpowdered refractory insulation material and Aby means of a diameterreducing process applied to the said s heating unit, compressing theinsulation to a hard dense mass so that the said mass may readilyconduct heat from the said resistance elements to l the said sheath,fitting the said end cap over a recessed portion of the said outersheath and securing the said cap to the said recessed portion to makethe unit waterproof.

LORET'I'A M. DALY.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS

